1. Field of the Invention
The present invention relates to the field of precious metal recovery from ore. More specifically, it relates to an improved method for recovering gold and other precious metals contained in ore in an amount not previously attainable.
2. Description of the Prior Art
Standard recovery techniques used to recover gold and other precious metals (such as silver, platinum, iridium, rhodium, palladium, osmium, and ruthenium, hereinafter incorporated in the use of the term "gold") from an ore usually proceed by separating organic material from the metals by thermal or mechanical means, followed by chemical dissolution of the gold as either the tetracyanide complex, the tetrachloride complex, or sometimes as the EDTA complex. Typical examples of ore recovery using cyanide include U.S. Pat. No. 4,188,208 to Guay and U.S. Pat. No. 4,401,468 to Henderson.
In many gold containing ores, minerals such as titanates, alumina silicates, or iron/aluminum silicates are complexed with the gold. In these cases, the availability of gold to the technique of chemical oxidation followed by dissolution is minimal. (While the Guay patent cited above discusses oxidizing carbonaceous ores with air followed by chlorine, such a process has little effect on these types of ore and generally gives poor results.) For example, gold complexed with alumina has an effective redox potential for oxidation of greater than 1.3 volts. This is the effective potential necessary to free the gold from the alumina before dissolution by cyanide or chloride is possible. Since the effective oxidation potential of free cyanide is only 0.9 volts, and the oxidation potential of hypochlorite is only 1.1 volts, neither can dissolve the gold from the alumina. In the case of titanium silicates, the gold can be even more bound, requiring approximately 1.9 volts. In other cases, the presence of free iron in the Fe.degree. state, or as ferrous metal complexes, changes the chemistry from simple oxidation of gold to the gold/iron couple, thus preventing gold recovery. Basically, gold is oxidized by cyanide, then is reduced by oxidizing the iron, and then is oxidized. Unless the gold is present at greater concentrations than the iron, or some other technology is available, the gold is unrecoverable. Large amounts of gold remain trapped in the ore with a resultant loss, given the high price of gold, of a dramatic amount of economic return.
In addition, intense roasting of many ores can cause the actual loss of so called micron gold. The boiling point of gold is sufficiently high that roasting does not actually vaporize the gold; however, very small particles of gold can be lost as an aerosol. Unless electrostatic precipitation apparatus are installed, and particulates are recovered with great care, as much as 75% of all gold in a particular type of ore can be lost.
Accordingly, there exists a clear need for a method of recovering a larger proportion of the precious metals contained in ore at an economic cost that has no detrimental side effects or results.
Ozone has long been used in precious metal mining in the destruction of the cyanides used for leaching the metals from the ore. Ozone, an allotrope of oxygen, is a pale blue gas, irritating to the nose and mucous membranes, with the formula O.sub.3. It is a very highly reactive molecule, decomposing upon reaction to normal diatomic oxygen, as well as to atomic oxygen, a very powerful chemical oxidizing agent. The history of ozone started in the late 1850's with the development of the many electrical experiments done at that time. The uses of ozone have been experimentally tested for approximately 100 years, with most of the potential uses still awaiting discovery. The oxidation potential of ozone is about 2.07 volts depending on the pH and other chemical properties of the reaction, or about 47 kcal/mole. Since the bond strengths of most chemical bonds in molecules are only about 25 to 35 kcal/mole, ozone is potentially able to react by bond breaking with most molecules. The magnitude of this reactivity is the major benefit of using ozone for chemical and biological purposes.
Two methods of commercially producing ozone and other reactive oxygen molecules currently exist. The first method is the spark source generator. Such a generator relies on the passage of high energy electrons from a corona discharge to react with diatomic oxygen in air. Some of the oxygen in the air is converted to atomic oxygen. The oxygen atoms then can react with diatomic oxygen to produce ozone. Despite the large capital costs and the sometimes complex technology, most commercial ozone production installations have been of the spark source type and this is the type of ozone currently used for cyanide destruction in precious metal recovery.
In contrast to the energy intensive spark source ozone generators, a second method of ozone production is possible. It has long been known that a layer of ozone resides near the top of the atmosphere. This ozone is produced by a photochemical reaction between the intense ultraviolet radiation of the sun and the oxygen of the atmosphere. Ultraviolet light creates ozone by the same mechanism as the spark source generators, namely the reaction of diatomic oxygen with UV light producing atomic oxygen. This atomic oxygen then reacts with diatomic oxygen to produce ozone. In addition to the ozone produced, several other reactive species are produced in the discharge that are also of use in commercial applications. This group of compounds produced by a UV ozone generator is sometimes referred to as activated oxygen. One has to be careful, however, that the design of a UV ozone generator is such that the oxygen is not overly exposed to the UV light because the same UV irradiation that produces the ozone also destroys it. In addition, the wavelength used in the generator should be less than 200 nanometers to avoid destroying the ozone.
At the present time, UV produced ozone is used almost exclusively for aqua filtration and pool and tub disinfection because of its favorable germicidal effect. Examples of such use are set forth in U.S. Pat. Nos. 4,517,084 to Pincon, 4,230,571 to Dadd, 4,189,363 to Beitzel, 4,179,616 to Coviello et al., and 3,336,099 to Czulak et al. UV ozone has not been used in the recovery of precious metals.